It could happen to anyone. We all know someone who has made this mistake. Never let it happen again! It is much easier to avoid the mistake than to repair the damage.

Problem: Inadvertently make the mistake of overlooking a Birthday, Anniversary or other date regarded as sacred by your wife or significant other. This mistake or oversight may be the result of an unexpected event, development, act of god or some other contributor beyond your control such as advancing age. Regardless of the cause, the result is often the same. "Serious Trouble". But then again, if you get into trouble for forgetting a conmemorative day, the value system of that person is not much of a value system to begin with.
Solution: This is really pretty simple. Just let it happen once. By the time the whole thing has settled down, believe me; you will never let happen again. You will have received such intense (unsolicited) training on the importance of keeping track of such dates and events it would be almost "impossible" to find yourself in this situation again. (Remember, a cat only has so many lives to live.)

For best results, you want to remember her birthday before her birthday. If you set up a reminder system, make the first notification early enough to give you time to act. If you use an electronic PIM, set reminders for two weeks before the birthday (to figure out what to get or do for her) and one week out (to double check that you've done it). Buying flowers and trying to make a dinner reservation on the commute home just won't cut it.
If you keep a list of things she admired or mentioned wanting during the year, you'll have an easy time at Christmas or her birthday. In addition, you get bonus points for having listened, and for getting her something she really wanted.

For best results, never let your spouse know you needed a reminder. She's much happier believing your mind works like hers, and is wired to remember things like this, rather than Sandy Koufax's lifetime ERA, or the goal Manchester scored five years ago.